It is of particular import to be able to determine the presence of a defect via nondestructive testing means in many applications. In manufacturing processes, the ability to perform nondestructive testing of manufactured articles avoids waste and costs associated with destructive testing. Moreover, destructive testing of components only provides statistical probabilities of component integrity for actual articles shipped since it is apparent that when destructive testing is employed, only non-tested articles are delivered to a customer. The ability to perform nondestructive testing provides the option of testing all manufactured articles yielding such benefits as improved product integrity and greater customer satisfaction.
Of present interest are nondestructive test techniques employing acoustic emission analysis of a workpiece under test. Typically, elastic waves are either induced in a workpiece or are inherent in a particular process. Elastic waves in a workpiece may interact with a defect to produce a change in an output signal from an acoustic sensor in elastic wave transmitting contact with the workpiece.
One method and apparatus for inspection of ceramic parts, and more specifically posistors, is disclosed in U.S. Pat. No. 4,277,977 to Lubitz, et al. Lubitz discloses that a voltage may be applied to a ceramic element and that, as a consequence of the applied voltage, temperature oscillation will occur in the vicinity of a pre-existing defect producing acoustic or sound pulses due to oscillatory abrasion of defect or fissure surfaces. The acoustic or sound pulses are detected via acoustic emission analysis techniques to produce an output signal indicative of the presence of the defect.
U.S. Pat. No. 4,086,817 to Jon, et al. recites a method and apparatus applicable for determination of weld integrity employing acoustic emission analysis. During AC welding operations, stress waves are generated and are emitted during time periods when the AC power is on, during a post-weld time period and during a third time period comprised of all the time intervals in which the absolute magnitude of each of the energy pulses is decaying. By analyzing acoustic emissions corresponding to stress waves emanating from the weld site, the integrity of the weld site may be ascertained.
Co-filed application Ser. No. 518528, of the same inventor and assignee as the present invention discloses a probe for inducing elastic waves in a workpiece and producing an output signal indicative of a defect.